Fist of all, let's talk about EncFS. EncFS is a free FUSE-based cryptographic filesystem. The encryption used depends on the installed encryption libraries on your device, AES should be no problem. EncFS uses a (typically hidden) directory where it stores your encrypted files, so setting up encryption with EncFS on your Nokia N900 is a really easy story.

Install EncFS, e.g. with

~ # apt-get install encfs

Create the directory where your encrypted files shall be stored, e.g. with

~ $ mkdir /media/mmc1/.crypto

Create the 'directory' where you want to access your decrypted files, e.g. with

~ $ mkdir /home/user/MyDocs/Crypto

Let EncFS create the encrypted filesystem with

~ $ encfs {encrypted storage} {mountpoint}

Example:

~ # encfs /media/mmc1/.crypto /home/user/MyDocs/Crypto -o allow_other

Choose the way you'd like to have your files encrypted

Choose a password for your EncFS

Enjoy your crypto-fs and unmount it with

~ # fusermount -u {mountpoint}

when you're done

I decided to publish this little tutorial even if it's incomplete. My N900 is broken (physically) and I don't think that the damage can be fixed. It crashed onto the floor with the charger plugged in. Its µUSB jack has been ripped off the PCB. Rest in peace my beloved friend.